1. Field of the Invention
This invention is concerned generally with an automatic control system for a clutch for coupling two rotatable shafts, and is more particularly directed to an automatic control system for a clutch for an automotive vehicle for coupling between a driven shaft, associated with the front axle for example, and a driven shaft associated with the back axle, so as to apply four wheel drive momentarily and automatically to the vehicle, especially when the front and rear wheels are rotating at different speeds.
2. Description of the Related Art
French Patent No. 2 135 983 describes a torque convertor for a four wheel drive automotive vehicle, operable to allow the engine torque to be divided differentially between the front and rear axles, when a difference in speed occurs between the front and rear wheels. This torque convertor comprises a hydraulic sliding coupling, which produces a torque as a result of a predetermined speed difference. This torque causes the axial displacement of a ring, which in turn acts on the pressure plate of a multi disc clutch mounted between the two shafts.
However, the torque produced by the hydraulic sliding coupling effects direct control of the ring which acts on the clutch, that is to say the hydraulic sliding coupling must itself ensure control of the clutch, and must be capable of transmitting relatively large torques.
Another disadvantage lies in the fact that control of the multi disc clutch is obtained by means of rollers which are displaceable on ramps for causing the displacement of the roller which acts on the pressure plate of the clutch. Such a structure is complex by nature, and requires accurate machining of the ramps so as to prevent any possible jamming of the rollers.
The invention seeks to overcome these disadvantages, and with this in mind proposes generally a system in which, for the mechanical control means, there is substituted hydraulic control of the coupling clutch; at the same time other advantages are obtained.